1. Field of the Invention
This invention relates to rubber compositions and particularly to rubber compositions having high reversion resistance. More particularly, this invention relates to reversion resistant rubber compositions which are useful as carcass compounds for aircraft tires.
2. Description of the Prior Art
Natural rubber is widely used as the carcass rubber in aircraft tires. While synthetic rubbers dominate the passenger car tire market, the properties of sulfur vulcanized natural rubber compositions (or compounds) including high abrasion resistance, good fatigue resistance and the ability to receive a retread, make natural rubber compositions uniquely suitable for aircraft tire carcass rubbers. Accordingly, natural rubber remains the rubber of choice for this purpose.
Aircraft tires are typically retreaded several times before the tire is discarded. The tread compound will typically wear out in about 200 to 300 landings (roughly 60 to 90 days of average aircraft usage). If the carcass is still sound, which can be ascertained by tests known in the art, the tire is retreaded.
Heat buildup occurs during each landing. Such heat buildup causes devulcanization or reversion of natural rubbber to a sticky, non-crosslinked state in the absence of an anti-reversion agent.
Anti-reversion agents for rubber compositions are well known in the art. These agents are particularly useful in natural rubber compositions, since natural rubber has a greater tendency to devulcanize under heat than do most synthetic rubbers.
U.S. Pat. No. 5,302,315 discloses, as vulcanization activators and materials for retarding devulcanization of rubber, a mixture of zinc salts of (i) one or more aliphatic or cycloaliphatic carboxylic acids containing 6-18 (preferably 8-12) carbon atoms and (ii) an aromatic (preferably monocyclic aromatic) acid. Example 1 of the patent discloses a zinc salt mixture which is the reaction product of 1.0 mole of zinc oxide, 1.4 moles of a C.sub.8 -C.sub.10 coconut fatty acid and 0.6 mole of benzoic acid, having a drop point of 100.degree. C.
PCT International Publication No. WO92/07904 (published application No. PCT/EP91/02048) discloses certain bis-citraconic imides as anti-reversion agents for rubber.